Michel Garçon – SPhN/Saclay – SIR2005 Workshop (Jefferson Lab, May 2005)

Generalized Parton Distributions:

the present program at Jefferson Lab

High luminosity compensates relatively low energy (for exclusive measurements, same Q2 as high energy machines are achieved) Large x range (.15 - .75) above resonance region into DIS

High resolution truly exclusive measurements

Experimental equipment from high resolution to large acceptance spectrometers, polarized targets, new electromagnetic calorimeters, highly polarized electron beam (80%)

☺ All this enhanced in the planned 12 GeV upgrade

The specificities of JLab

W = 2.8

2.4

2 1.8 GeV

ep→epγ (DVCS) BSA CLAS 4.2 GeV Published PRL

CLAS 4.8 GeV Preliminary

CLAS 5.75 GeV Preliminary

(+ σ) Hall A 5.75 GeV Fall 04

CLAS 5.75 GeV Spring 05

ep→epγ (DVCS) TSA CLAS 5.65 GeV Preliminary

e(n)→enγ (DVCS) BSA Hall A 5.75 GeV Fall 04

ed→edγ (DVCS) BSA CLAS 5.4 GeV under analysis

ep→epe+e- (DDVCS) BSA CLAS 5.75 GeV under analysis

ep→epρ σL CLAS 4.2 GeV Published PLB

CLAS 5.75 GeV under analysis

ep→epω (σL) CLAS 5.75 GeV Accepted EPJA

+ other meson production channels π, η, Φ under analyses in the three Halls.

GPD Reaction Obs. Expt Status

),,( tH From

ep → epX

Dedicated set-up

),,(~ tH ),,( tE

),,( txH

x

duEH )2(,

x

duEH )2(,

)( du

DVCS and GPDs : Beam and target spin asymmetries

DVCS-BH interference generates abeam spin cross section difference:

and atarget spin cross section difference:

sinA

EHH

)(4

~)()(

2)( 22211 tF

MttFtF

xxtFA

B

B

q

qqq tGPDtGPDe ),,(),,(

~ 2 E),H(H,

Spin asymmetries:

TSABSA /

sin'A

EHH

221211 4222

~' F

MtFx

xxFF

xxFA B

B

B

B

B

CLAS/DVCS at 4.2 and 4.8 GeV:ep→epγ from analysis of ep→epX spectra

Published measurement at 4.2 GeVPhys.Rev.Lett.87:182002,2001

2 2

2

1.25 GeV0.19

0.19 GeVB

Qx

t

Preliminary CLAS analysiswith 4.8 GeV data (G. Gavalian)

Preliminaryγ

π0

CLAS: high luminosity run at 5.75 GeV

First JLab experiment with GPDs in mind

(october 2001 – january 2002)

- polarized electrons, E = 5.75 GeV

- Q2 up to 5.5 GeV2,

-Integrated luminosity: 30 fb-1

- W up to 2.8 GeV

W = 2.8

2.4

2 1.8 GeV

0.15 < xB< 0.41.50 < Q2 < 4.5 GeV2

-t < 0.5 GeV2

PRELIMINARY

(not for circulation)

H. Avakian & L. Elouadrhiri

GPD based predictions(Vanderhaeghen)

0 are suppressed due to analysis cuts (only low t),residual contribution (π/γ ~ 5-15%) estimated from MC

CLAS/DVCS (ep → epX) at 5.75 GeV

PRELIMINARY

(not for circulation)

CLAS/DVCS (ep → epX) at 5.75 GeV

t – dependence of BSA for photon and pion production:

PRELIMINARY

(not for circulation)

0 asymmetry (two photons required)

Exclusive ep ep

S. Chen

A

5.65 GeV run with NH3 longitudinally polarized target, Q2 up to 4.5 GeV2

DVCS with a polarized target in CLAS

* Detect all 3 particles in the final state (e,p,γ) to eliminate contribution from N (but calorimeter is at too large angles) ,

* Apply kinematical cuts to suppress ep→epπ0 contribution.

* Remaining Φ-dependent π0 contribution (10-40%) extracted from MC.

* π0 asymmetry measured

PRELIMINARY

(not for circulation)

PRELIMINARY

(not for circulation)

DDVCS(Double Deeply Virtual Compton Scattering)

γ*T γ*T

M. Guidal & M. Vanderhaeghen, PRL 90A. V. Belitsky & D. Müller, PRL 90

The (continuously varying)virtuality of the outgoing

photon allows to “tune” thekinematical point (x,ξ,t) at

which the GPDs are sampled (with |x | < ξ).

e- e+

e-

p p

e- ),),',((~Im tqxHT DDVCS

DDVCS-BH interference generates abeam spin asymmetry sensitive to

DDVCS: first observation of ep → epe+e- in CLAS

* Positrons identified among large background of positive pions

* ep→epe+e- cleanly selected (mostly) through missing mass ep→epe+X

* Φ distribution of outgoing γ* and beam spin asymmetry extracted(integrated over γ* virtuality)

A problem for both experiment and theory:

* 2 electrons in the final state → antisymmetrisation was not included in calculations,

→ define domain of validity for exchange diagram.

* data analysis was performed assuming two different hypotheses

either detected electron = scattered electron

or detected electron belongs to lepton pair from γ*

Hyp. 2 seems the most valid

→ quasi-real photoproduction of vector mesons

but…

Lepton pair squared invariant mass

S. M

orro

w &

M. G

arço

n

Deeply virtual meson production

Meson and Pomeron (or two-gluon) exchange …

… or scattering at the quark level ?

π, f2, Pρ0 (σ), f2, P

ω π, f2, P

Φ P

Flavor sensitivity of DVMP on the proton:

ω

ρ0 2u+d, 9g/4

ω 2u-d, 3g/4

Φ s, g

ρ+ u-d

γ*LωL

6

2

4

),(),,)((1)(1Q

tfdxdztxbEaHixz

zQQdt

d MSL

(Photoproduction)

Exclusive ρ meson production: ep → epρ

CLAS (4.2 GeV)

Regge (JML)

C. H

adjid

akis

et a

l., P

LB 6

05

GPD formalism (beyond leading order) describes approximately data

for xB<0.4, Q2 >1.5 GeV2

GPD (MG-MVdh)

CLAS (5.75 GeV)

Analysis

in progress

Two-pion invariant mass spectra

L. Morand et al., hep-ex/0504057, to be published in EPJA

Analysis of ω polarization from ep → epπ+π-X configurations (for the first time for this channel above Q2 ~ 1 GeV2)

• Cross sections larger than anticipated at high t (see J.-M. Laget, PRD 70, 054023)• SCHC does not seem to hold → not possible to extract σL

handbag diagram estimated to contribute only about 1/5 of measured cross sections→ ω most challenging/difficult channel to access GPD • Evidence for unnatural parity exchange 0 exchange very probable even at high Q2

Deeply virtual ω production at 5.75 GeVAnalysis of cross sections from ep → epπ+X configurations

JLab/Hall A JLab/CLASCalorimetrer and supraconducting magnet

within CLAS torus

ee’p

γ

Dedicated, high statistics, DVCS experiments

→ Detection of 3 particles e, p and γ in final state→ Firmly establish scaling laws (up to Q2 ~ 5 GeV2), if observed, or deviations thereof understood, first significant measurement of GPDs.

→ Large kinematical coverage in xB and t leads to femto-tomography of the nucleon→ Opens the way for an ambitious program

with JLab@12GeV (CLAS12 and other)

JLab dedicated DVCS experiments in 2004 - 2005

Experiment completed (Sept-Nov. 2004)

PbF2 electromagnetic calorimeterFast-digitizing electronics→ analysis of double pulses (pile-up)

First

double

and

triple

coincidences !

(luminosity = 1037 cm-2s-1)

HMS•calo

HMS•calo•scint.

Scintillator array for proton detection

(P. B

ertin

, C.E

. Hyd

e-W

right

, R. R

anso

me,

F. S

abat

ié, e

t al.

CEB

AF/

E-00

-110

)

EHH

)(4

~)()(

2)( 22211 tF

MttFtF

xxtFA

B

B

DVCS on the neutron

DVCS-BH interference generatesa beam spin cross section difference

Main contribution for the proton Main contribution

for the neutron

(P. B

ertin

, C.E

. Hyd

e-W

right

, F. S

abat

ié, E

. Vou

tier e

t al.

CEB

AF/

E-03

-106

)

→ (within a model) Sensitivity to quark angular momentum J

sinA

Veto detector added to the p-DVCS set-up

Experiment completed (Nov.-Dec. 2004)

Supraconducting solenoid

Inner calorimeter (PbWO4)

CLAS/DVCS

424 crystals, 16 mm long, pointing geometry, ~ 1.2 degree/crystal,

APD readout

Photon detection in IC and EC (view from target)

Calibration from π0→γγ

σ = 7.5 MeV

Mγγ (GeV)

η

Dependence of asymmetryand total cross-section asa function of xB, t, Q2 , bins

Projected results (sample)

E01-113 V. Burkert, L. Edouardrihi, M. Garçon, S. Stepanyan et al. Run March-May 2005

Full exclusivity from 3-particle detection

Hall A

CLAS (from preliminary analysis of 2-hour run)

All (eγp) events(eγp) events after kinematical cuts

Conclusions and outlook

Jefferson Lab is playing a leading role in providing

the experimental basis of the GPD concept.

Once proper scaling laws are verified in DVCS (and possibly DVMP),

first significant constraints on GPD models from dedicated experiments.

A complete mapping and measurement of GPDs (from the quark sea to the valence region)

will probably have to await the 12 GeV upgrade.